Fault interrupting devices function to isolate a fault condition in a power distribution system. Upon clearing of the fault condition some fault interrupting devices are also operable to reclose the circuit. Faults in a power distribution system can occur for any number of reasons and are often transient. Detection and isolation of the fault mitigates damage to the system as a result of the fault. An ability to reclose the circuit following a fault without replacement of hardware components allows the power distribution system to be returned to normal operation quickly, and in some instances, without operator intervention.
Combined fault interrupting and recloser devices may be designed to operate or be operated after a fault interruption to reclose the faulted line or lines. Following reclosing, if the fault is not cleared the device will detect the fault and again operate to open the circuit to isolate the fault. When a fault is determined to be permanent, the fault interrupting device should act to isolate the circuit and prevent further reclosing attempts.
Several types of fault interrupting and reclosing devices incorporate vacuum interrupters to perform the circuit interrupting and subsequent reclosing functions. During current interrupting operation, as the vacuum interrupter contacts open, there is redistribution of material from the contacts to the other surfaces within the interrupter. Contact material redistribution occurs with each operation, and therefore, the vacuum interrupter is capable only of a finite number of fault current interrupting operations. The number of fault interrupting operations may be specified for a particular fault protection device based upon design information and intended application. The fault interrupting and reclosing device may include a counter to track the number of operations.
The vacuum interrupters in fault interrupting and reclosing devices are capable of operating very quickly under the action of a drive mechanism, such as a drive solenoid. Operation in the presence of an asymmetric current can expose the contacts to large arcing time, for example, arcing times in excess of 10 ms. Such long arcing times have the potential to seriously degrade the life of the fault interrupter and reclosing device.
In practice, therefore, the actual number of interrupting cycles a vacuum interrupter is capable of, and hence the fault interrupting and reclosing device incorporating the interrupter, depends on a number of operating characteristics including characteristics of the interrupted fault current and the operating characteristics of the vacuum interrupter. For example, material erosion and corresponding contact degradation become significantly more pronounced with the magnitude and asymmetry of the interrupted current. The number of cycles defining the life of the fault interrupting device is conservatively set to ensure the proper operation of the device throughout its specified life and over its rated current interrupting capacity.